Interface Engineering of Inorganic Thin-Film Solar Cells �� Materials-Science Challenges for Advanced Physical Concepts

PROGRESS

REPORT

www.advmat.de

4196

Interface Engineering of Inorganic Thin-Film Solar Cells– Materials-Science Challenges for Advanced PhysicalConcepts

By Wolfram Jaegermann,* Andreas Klein, and Thomas Mayer

The challenges and research needs for the interface engineering of thin-film

solar cells using inorganic-compound semiconductors are discussed from a

materials-science point of view. It is, in principle, easily possible to define

optimized device structures from physical considerations. However, to realize

these structures, many materials’ limitations must be overcome by complex

processing strategies. In this paper, interface properties and growth

morphology are discussed using CdTe solar cells as an example. The need for

a better fundamental understanding of cause–effect relationships for

improving thin-film solar cells is emphasized.

1. Introduction

Thin-film solar cells based on highly absorbing compoundsemiconductors have been introduced as a promising alternativefor economic, competitive, photovoltaic (PV) energy sources. For along time this promise was not fulfilled as Si was dominating themarket with a steady and considerable cost reduction following thewell-known scale of market law.[1] Recently, however, the projectedbright future seems to be within reach as a considerable marketshare has been obtained lately and promising expectations havebeen quoted.[2] In addition, new materials and concepts are beingintroduced, ranging from injection-type solar cells in organic/inorganic hybrid structures,[3,4] to organic or polymer-baseddevices,[5] and finally to third-generation concepts,[6] for whichthe original Shockley–Queisser limit derived for a PV-absorberwith only one bandgap is overcome.[7]

The basic idea of third-generation concepts is to reducethermal and optical losses in solar cells. Using, for example,multiabsorber layers, intermediate bands, or multicarrierformation, it is possible to reduce the principal energy lossesof photovoltaic energy conversion involving only one absorberbandgap, Eg (no photon absorbed for hn<Eg; thermal losses ofexcess energy for photons hn>Eg). With the exception ofmultijunction cells, no working devices have been realized yet.

[*] Prof. Dr. W. Jaegermann, Prof. Dr. A. Klein, Dr. T. MayerSurface Science Division, Institute of Materials ScienceDarmstadt University of TechnologyPetersenstr. 32, 64287 Darmstadt, (Germany)E-mail: [email protected]

DOI: 10.1002/adma.200802457

� 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Promising, all-thin-film, non-epitaxial mul-tijunction solar cells are not within reachdue to the lack of promising thin-filmabsorber layers, besides amorphous/micro-crystalline Si cells.[8,9] As we cannot addressall of the interesting subjects related to PVenergy conversion within a short reviewarticle, we will concentrate on inorganicthin-film solar cells and want to discusssome key issues of research and develop-ment. A good summary of the differentenergy technologies is given in ref.[8]

Related to thin-film solar cells, it isinteresting to note that very few inorganic

semiconductors have been developed to commercialization:microcrystalline and amorphous Si,[9] the CuInSe2 (CIS)chalcopyrite family,[10] and CdTe.[11,12] We will not consider thehighly efficient, epitaxial, multiabsorber structures based on 3-5semiconductors, mostly used for space application and concen-trating devices, as they have different research needs.[13]

However, current thin-film technologies may not be ideal anddo not provide long-lasting solutions due to limitations related toprocessing and availability of materials. The latter will not be aproblem for a solar-cell production volume in the gigawatt range,but for a significant substitution of primary energy sources bysolar cells, an overall production in the terawatt range is needed inthe long range, which may not be possible with somematerials.[14] On the other hand, there are many alternativecompound semiconductors known that have promising bulkproperties, such as Cu2S, Zn3P2, Bi2S3, FeS2,

[15] which, in spite ofconsiderable research in the past, have not made it to the neededmaturity level of technology. The success of CIS and CdTe waspaved by either accidentally or empirically found processing‘‘tricks’’, such as Na diffusion from the glass for CIS and theCdCl2 activation treatment for CdTe,[16,17] which have lead to thematurity of these systems today.

Our emphasis in this article will be to present our view on thecommon understanding and still-given research needs ofadvanced thin-film solar cells using compound semiconductorslike CIS and CdTe, and also to discuss the limitations and chancesfor novel materials. Based on our expertise, we will mostlyaddress interface phenomena, which are of crucial importancefor thin-film solar cells. We will first present an idealized thin-filmsolar-cell device structure based on simple physical considerationand afterwards want to discuss the materials-science challengesin order to realize such devices, mostly based on our own actualresearch focus on CdTe solar cells.

Adv. Mater. 2009, 21, 4196–4206

PROGRESS

REPORT

www.advmat.de

Wolfram Jaegermann studiedChemistry at the University ofDortmund and received hisPh.D. in Physical Chemistryfrom Bielefeld University. Hehas studied interface propertiesof energy-conversion systems,beginning in 1982 at theHahn–Meitner Institute inBerlin and became FullProfessor and chair for SurfaceScience in theMaterials Science

department at TU Darmstadt in 1997. His current activitiesare around materials and interface studies of energy-conversion and storage systems.

Andreas Klein studied physicsat the Universities of Tubingenand Konstanz. After his Ph.D. atthe University of Konstanz, heworked as a research scientistat the Hahn–Meitner Institute.He joined the surface-sciencegroup at TU Darmstadt in 1998and became associate Profes-sor in 2008. His research topicsare thin-film solar cells, trans-parent conducting oxides and

ferroelectric materials, with a focus on the electronicproperties of materials and interfaces.

Thomas Mayer studied SolidState Physics at the Universityof Technology in Berlin. Afterhis doctorate at the Hahn–Meitner Institute, he joined theSurface Science group at TUDarmstadt. He has extendedthe application of synchrotronphotoelectron spectroscopy tosolid/liquid interfaces as modelelectrolytes deposited in situand ex situ liquid treatments.

Currently he is developing inorganic/organic and organic/organic interfaces and composites for photovoltaic applica-tions in dye-sensitized and bulk-heterojunction solar cells.

Figure 1. Basic working principle of a solar cell under illumination (top)and its realization by a p/n homojunction (middle) and a p/n heterojunc-tion (bottom). Band energy diagrams are shown for illuminated cells.

2. Idealized Solar-Cell Structures Derived fromPhysical Considerations

In principle, light-induced electrons and holes have to propagateto two separate electrical contacts in a solar cell. Physically, thisproceeds in three major steps: i) efficient generation ofelectron-hole pairs with large differences in their chemicalpotentials (difference in the energy position of the quasi Fermilevels, EF,n�EF,p, for electrons and holes) by absorption ofsunlight (for optimized light harvesting, an optical gap of theabsorber material in the range of 1–2 eV is required); ii) efficient

Adv. Mater. 2009, 21, 4196–4206 � 2009 WILEY-VCH Verlag G

charge-carrier separation in the bulk of the absorber material,which needs travelling paths of electrons and holes exceeding thethickness of the absorber layer; iii) efficient injection of thecarriers into the contacts, which needs suitable barrier heights atthe interfaces. The three requirements can be met by differentdevice structures, such as p-n homojunctions or p-n heterojunc-tions, which may be used depending on the physical properties ofthe materials combined in the solar cell (see Fig. 1).[18]

Homojunctions are limited in their perspectives for highlyabsorbing semiconductors due to their expected front-contactrecombination and, in addition, require a material that can bedoped as both n- and p-type, which is possible for Si and most 3-5semiconductors. The latter have not been able to be used inpolycrystalline thin-film cells so far due to the high recombina-tion rates of photoexcited carriers at the grain boundaries. Grainboundaries of 2-6 and derived chalcopyrite semiconductors are,in contrast, surprisingly passive. But, as a result of intrinsicdefect-formation processes (self-compensation),[19,20] the 2-6compounds are usually intrinsically doped and the control ofthe doping is rather limited. In particular, p-n homodiodes canhardly be realized with these materials. As a consequence,polycrystalline-compound semiconductor solar cells employ a p-nheterojunction device concept, where only a single type of dopingis required for either material forming the junction. Doping ofeither n- or p-type to a high level, which is required in standardsemiconductor technology for electrical-contact formation,[21] canalso be prohibited with 2-6 compounds.[19,22] Thus, additional

mbH & Co. KGaA, Weinheim 4197

PROGRESS

REPORT

www.advmat.de

Figure 2. Schematic energy diagram of advanced n-i-p devices in the dark(top) and under illumination in open circuit conditions (middle). The effectof interface states and band discontinuities are illustrated in the bottomgraph.

4198

materials, like transparent conducting oxides (TCOs) (see, forexample, ref.[23]), have to be included. Therefore, the hetero-junction device concept is presently used for thin-film compoundsemiconductor solar cells. Inevitably a number of interfacesbetween materials with different structures and/or latticeconstants are introduced, which cause a considerable complexityin the device formation, as is described inmore detail in Section 4using CdTe as an example.

An alternative, quite-generally applicable device concept hasbeen suggested as the n-i-p junction shown in Figure 2.[18,24] Theenergy-band diagram of a (non-illuminated) idealized solar cell isshown in the upper diagram of Figure 2. The absorber layer,which needs no doping, is situated between the highly dopedwide-bandgap contact layers, preferentially consisting of highlydoped large-energy-gap materials like transparent conductiveoxides. These serve as ‘‘membranes’’ for the other type of chargecarrier; that is to say, there is a large barrier in either the valence orconduction band at the interface to the absorber layer, whicheffectively prevent holes or electrons from entering the respectivecontact material. The large energy gap also allows fortransmittance of large parts of the solar spectrum.

The thickness of the absorber layer, d, needs to exceed theabsorption length (3a�1 with a the absorption coefficient) toabsorb all of the incoming light with energy larger than thebandgap of the absorber. This translates for most highlyabsorbing compound semiconductors (direct-band semiconduc-tors) to a thickness in the range below 1–2mm. For efficientdevices, this thin absorber layer must be prepared with acontrolled film morphology to achieve compact and pinhole-freelayers in order to avoid short circuits, low shunt resistances andweak diodes.

A solar cell under illumination and in open-circuit conditionsis shown in the middle diagram of Figure 2, including thesplitting of the quasi-Fermi levels of electrons and holes withinthe absorber layer. The expected open-circuit voltage,Voc, given bythe difference of the quasi-Fermi levels within the absorber bulk,is considered to bemaximized for such an n-i-p device, as the darkreverse current, j0, is minimized and the photocurrent, jph,maximized for the given conditions and negligible interfacerecombination.

However, these idealized conditions may not be valid,depending on the given material combinations. Interface recom-bination can only be expected to be small for interfaces free ofdefect states. The main factor influencing interface recombina-tion is the density of interface states at the phase boundary, whichwill be low for lattice-matched heterointerfaces of materials withsimilar bonding characters, such as at GaAs/AlAs or InP/CdSinterfaces.[25] For most cases that are expected in thin-film solarcells, the combination of dissimilar materials will lead to a highinterface-defect density of states and, as a consequence, to highrecombination velocities. Therefore such heterojunctions in n-por n-i-p devices will only work for a proper electronic passivation(interface engineering) of the defect levels. To understand theapplied interface-engineering effects in given devices is still amatter of research and also a challenge if new materialcombinations are to be considered.

In addition, a proper alignment of the energy levels (valenceand conduction band offsets (DEVB and DECB) across the phaseboundaries must be assured. The band edge alignments are

� 2009 WILEY-VCH Verlag Gmb

decisive factors, and are also objects of interface engineering: theyneed to be optimized with respect to the electronic properties ofthe dissimilar phase boundaries. In the upper and middlediagrams of Figure 2, optimized DEVB and DECB conditions are

H & Co. KGaA, Weinheim Adv. Mater. 2009, 21, 4196–4206

PROGRESS

REPORT

www.advmat.de

shown with no discontinuity for electrons to the nþ-dopedwindow layer and no discontinuity for holes to the pþ-dopedwindow layer. To the contrary, the DEVB and DECB values for theminority carriers (electrons/holes on the pþ- and nþ-doped TCO)are maximized to reduce the reverse current.

Ideal interface properties, that is to say, suitable bandalignments and a low defect density may not be expected withoutany additional measures involving appropriate adsorption orbuffer layers. In general, the natural situation may be morerelated to the case given in the bottom diagram in Figure 2,showing, for example, a ‘‘cliff’’ for the electron injection and a‘‘spike’’ for hole injection into the contacts. The cliff will lead toa loss of photovoltage,VOC, as the quasi-Fermi level will be pinnedby the Fermi level position of the contact and may not approachthe band edge of the absorber. The spike may lead to losses inphotocurrent as it works as a barrier for the charge carriersleading to unfavourable series resistances in the cell.

Overall, the materials-science challenges are to developappropriate interface-engineering procedures and competitivedeposition techniques for thin-film absorber and contact layers totranslate the physical device structure into the reality of anefficient thin-film solar cell. This is an ongoing research aim forgiven thin-film technologies, and even more so for new materialsor materials combinations. To meet the requirements, we haveproposed the integration of deposition techniques and surfaceanalytical techniques in the form of cluster tools for thin-filmsolar-cell research.

Figure 3. Darmstadt Integrated System of Solar-Energy Research (DAISY-SOL): schematic set-up of preparation cluster (top) and realization includ-ing surface-analysis system (bottom).

3. Experimental Approach using the DarmstadtIntegrated System for Solar Research –DAISY-SOL

As discussed above, the junction properties of semiconductordevices depend on the crystalline, chemical and electronicstructure of the interface. Depending on the given interfaceproperties, defect levels may be formed, which, even in thesub-monolayer regime, are sufficient to lead to Fermi levelpinning effects, inferring contact formation (for reviews onfundamental issues of contact formation in relation to solarcells, see for example ref. [24,26,27]), and also to fast surfacerecombination. For this reason, preparation of the films and thejunctions must be possible with maximum control, avoidingunwanted contamination. It should be noted at this stage that alsothe incorporation of extra elements, such as the effect due tocontact with air (surface oxidation or O inclusion into the bulk asdiscussed for CdTe solar cells), may have positive effects, but thisshould be possible in a controlled manner.[28,29]

In order to address interface-related issues of thin-film solarcells we have built a cluster tool specifically designed and used forthin-film solar-cell research. It consists of a number of depositionchambers directly coupled to a multitechnique, ultra-highvacuum (UHV) analysis system, integrating preparation, proces-sing and analysis. A schematic representation of the design and apicture of its realization are shown in Figure 3. The DAISY-SOLcluster tool (DArmstadt Integrated SYstem for SOLar Research) iscurrently directed to CdTe thin-film solar cells, reflecting today’smajor research effort of the group. However, the system can beeasily modified for other solar-cell materials. The independent


deposition chambers are based on a flexible design, and can beeasily modified, thus enabling step-by-step film and interfacegrowth under maximized experimental control (the chambersare based on UHV-technology). If necessary, the defined additionof extra chemicals can also be included. It can be used forfundamental studies, for example, with the help of the analysissystem (VG ESCALAB 250), providing state-of-the-art surfa-ce-analysis techniques, including monochromatized X-ray photo-electron spectroscopy (XPS), ultraviolet photoelectron spectro-scopy (UPS), low-energy ion-scattering spectroscopy (LEISS),sputter-depth profiles and laterally resolved XPS mapping. Inaddition, with the given deposition and processing chambers(closed-space sublimation (CSS), physical vapor deposition(PVD), magnetron sputtering, enhanced chemical vapor deposi-tion (CVD), wet processing, adsorption) all of the steps needed toproduce solar cells can be performed, which also allows themanufacturing of complete solar cells to be optimized. Thecircular transfer chamber offers the advantage that any depositionsequence can be realized: therefore substrate (back-contact down)as well as superstrate (TCO front-contact down) solar cells can beeasily prepared in the same set-up including, if needed or wanted,additional processing and deposition steps in-between. It is


PROGRESS

REPORT

www.advmat.de

4200

expected that, based on the obtained results, a better under-standing of the limitations and possible solutions of materi-al-related properties of thin-film solar cells will be gained and onthis basis, knowledge-based research and development will bepossible for further optimization and development of newmaterials.

4. CdTe Solar Cells as an Example of ResearchNeeds and Optimization Perspectives

As a review of the research needs and optimization perspectivesof all of the different, thin-film solar-cell types would extend thescope of the paper by a long way (interested readers may referto ref.[8]), we want to concentrate on two specific problems relatedto CdTe solar cells, which are discussed in a broader context inreference to other types of solar cells.

The band energy diagram is given in Figure 4; that which isshown was determined by our group for CdTe solar cells preparedby closed-space sublimation (CSS) following technology devel-oped by the ANTEC Solar Energy AG company.[30] It wasconcluded that similar energy band diagrams are also valid forother CdTe-production technologies followed by various groupsall over the world.[12,31–34] Based on this energy diagram, anumber of the main shortcomings of given CdTe devices can bededuced immediately.

The CdTe absorber layer is generally found to be only weaklyp-doped (in the range of 1013 to 1014 cm�3, mostly depending onthe use of Cu for the back contact, which diffuses into thematerialand forms acceptor states).[35–37] As a consequence, the n-CdS/p-CdTe heterojunction does not provide sufficient band bending(diffusion voltage) in the CdTe absorber layer, limiting themaximum photovoltage to be expected. In contrast to GaAs solarcells (VOC¼ 1040mV) with a similar bandgap,[8] CdTe solar cellsprovide only VOC values of 850mV (depending on Cu content),[38]

which is only slightly better than 50% of the band-gap value. Onthe other hand, layer thicknesses above 5mm, which are too large,do not allow for the formation of effective n-i-p junctions, asproposed above as the ideal thin-film solar-cell device structure:The given solar cell is thus a bad compromise for an n-pheterojunction and an n-i-p structure. Related problems using

Figure 4. Experimentally determined band-energy diagram of CdTe thin-film solar cells following ANTEC technology. Adapted from [30].


thinner absorber layers and possible solutions will be discussedin Section 4.2 below. A CdS buffer layer is needed for theoptimization of the front-contact interface properties (see Section4.1), but due to the rather-low bandgap of 2.4 eV, it acts as a filterfor high-energy photons of the solar spectrum and hardlycontributes to photocurrent generation.[34] As a consequence, thephotocurrents are lower than expected for a 1.5 eV absorber layerwith an energy gap of 1.5 eV as given for CdTe. Due to the highionization potentials of CdTe and Fermi level pinningeffects, there is always the tendency of a reverse barrier toform at the CdTe back contact, and good Ohmic back-contactbehaviour is hardly ever found. Limitations of the back contactand possible solutions will also be discussed in Section 4.1,which contains approaches to interface engineering of hetero-junctions in thin- film solar cells with special reference to CdTecells. Section 4.2 will cover the needed control of nucleation andgrowth of the different layers to be deposited on foreignsubstrates.

4.1. Interface Engineering in Thin-Film Solar Cells: CdTe

Junctions

In thin-film solar cells, a heterojunction is used to prepare thefront contact. As direct contact to the TCO layers leads to pinningeffects and recombination losses, buffer layers are included, suchas CdS in CIGS and CdTe solar cells.[34] These buffer layerscontribute to the passivation of the surface/interface statesformed from non-chemically saturated bonds, which are alsoexpected for abrupt junctions between lattice-mismatched anddissimilar materials. The interface properties of CdTe/CdSheterointerfaces prepared in situ before and after the CdCl2activation step have been investigated in detail by surface-sciencestudies and the results are summarized in Figure 5.[39–42] It isclearly evident from these results, in accordance with theliterature, that the CdCl2-activation treatment favours interdiffu-sion of CdS and CdTe at the phase boundary, which leads to areduction of the interface density of states at the phaseboundary.[43] As a consequence, Fermi level pinning as observedfor the abrupt junction and also the related surface recombination

Figure 5. Band energy diagram of CdS/CdTe interfaces prepared by PVDbefore activation (left) and after activation (right). Adapted from [41].


PROGRESS

REPORT

www.advmat.de

ECB

EF

p– semiconduct. metalE

Evac

tunnel contact(high doping level needed)

high p-dopedregion

holes

Fermi level pinning

EVB

p– semiconduct. metal

B

dipole potential

interface states

Alternative

Figure 6. Formation of metal back contacts according to the Bardeen limitwith Fermi level pinning (left). An opposing barrier to the rectifying frontcontact is formed. Formation of tunnelling contacts occurs, as expected forhighly pþ-doped CdTe back surface layers (right).

velocity are strongly reduced, and the solar cell shows stronglyincreased conversion efficiencies (therefore the high-temperaturetreatment with CdCl2 is named ‘‘activation’’ treatment). It shouldbe noted at this stage that the CIGS/CdS interface is not a simple,abrupt heterojunction but forms a wide-band intermediate(ordered-vacancy) layer that is responsible for the improvementof the solar-cell performance.[44–47] As a consequence, one mayconclude that a proper passivation of the interfacial electro-nic-defect levels is an essential precondition for the realization ofefficient thin-film solar cells with new materials, as discussedabove. The detrimental effects of interface states due to latticemismatch are well known and have been intensively discussed insemiconductor physics.[21,48] Rectifying contacts allowing highconversion efficiencies in thin-film solar cells will only be realizedif proper interface-engineering steps can be identified. These maybe interdiffused junctions, as in the case of CdTe, or properchemical passivation layers, such as for mc-Si/a-Si junctionswhere H-passivation of dangling bonds is operative.[49]

The back contact is considered to be another major limitationof CdTe solar cells. In the literature, very often, the high ionizationpotential of CdTe of 5.9 eV, given by the position of thevalence-band edge, is considered to be a problem as metalcontacts do not form an Ohmic contact for hole transport.[27]

However, this argument is probably not valid, as for mostwell-defined metal contacts prepared with covalent semiconduc-tors, the contact formation is not governed by the Schottkylimit.[50] In the Schottky limit, the barrier height for holes is givenby the difference between the metal work function and thesemiconductor ionization potential. For covalent semiconduc-tors, the Bardeen limit is usually observed in barrier formation,and is due to Fermi level pinning by the interface states.[51] This isalso the case for metal contacts on CdTe surfaces, as haspreviously been shown by electrical investigations and also byphotoemission studies.[52–55] We have also investigated a numberof different metals covering a wide range of work functions, buthave always observed a rather-similar value of the pinningposition. The observed pinning effects are schematicallysummarized in Figure 6 with similar barrier heights, FB, ofaround 1 eV for all of the metal contacts investigated, in goodcorrespondence to previous studies.[24,56] However, it must benoted that all of the investigated metals form reactive interfaces,which leads to a decomposition of the CdTe. Other authors havesuggested several different pinning positions scattered aroundFB¼ 1 eV that are not evident from our experiments.[55] Cdinterstitials formed during the decomposition reaction of theCdTe may diffuse into the near-interface region of the CdTebase material. The experimental and theoretical values forthese defects situated at 1.15 eV and 0.93 eV above thevalance-band maximum are in good agreement to the obtainedpinning position.[34,57]

Cu, or Cu-containing phases, have proven to be exceptionalin their contact properties to CdTe.[12,34] High conversionefficiencies and low back-contact resistances due to the absenceof back-contact opposing diodes, as for pinned contacts, arerealized only with such contacts. Using Cu deposited ontoCdTe films, it can by shown by XPS studies that a simple Cuoverlayer is not formed, rather that the Cu, at least in part,diffuses into the substrate, with the formation of metallic Cdresidues.[58] This interdiffusion, which is also evident in other


investigations,[36,37,59,60] can be enhanced by increasing thesubstrate temperatures to about 300 8C. It is interesting to notethat due to the temperature-activated interdiffusion, a change inthe barrier height from the original pinning position of 0.9 eVabove the valence-bandmaximum to a considerably lower value of0.6 eV is also found, which is evidently related to the formation ofCuxTey interface phases.[58] The advantageous effect of Cuinterdiffusion has already been known for some time and isconsidered to be the key point of producing low-resistance backcontacts by Cu, inducing strong p-doping of the CdTe and thusallowing for the formation of tunnel contacts (Fig. 6).[61] As Cueasily diffuses into CdTe as well as other 2-6 semiconductors,[59,62]

there are efforts to find alternative solutions for low-resistive backcontacts, such as doping with Sb or N. One very-promisingapproach follows the idea presented in Figure 2 using a ZnTecontact, which, itself, can be p-doped by Cu, N, or Sb.[63–65] Theband energy diagram follows the conditions as defined above (seeFig. 7);[63] however, the lattice mismatch of about 6% betweenZnTe and CdTe must also be electronically passivated as for thefront-contact CdS/CdTe, and this has not yet been optimized.Nevertheless, in principle, contact with ZnTe is promising forimproved CdTe solar cells.

It should be added that the back contact of CIGS solar cells,which contain a semiconducting MoS2 or MoSe2 layer formedduring deposition of the absorber layer onto the Mo-covered glasssubstrate, also forms an opposing barrier.[66] As the doping of theCIGS surface layer seems to be high enough, this barrier is onlyevident in electrical measurements at low temperatures.[67]

In summary, it can be emphasized that one of the majorchallenges for thin-film solar-cell research is to find a properalignment of the energy bands across the interfaces of app-ropriate heterostructure phases. As usually lattice-mismatchedor dissimilar materials are combined, Fermi level pinning and/or surface-recombination losses drastically reduce the efficiencyof the solar cells. For established solar cells, specific processing‘‘tricks’’ have been identified to overcome these problemsas discussed above. For new types of absorber materials,systematic studies of possible routes of interface engineeringto passivate surface/interface defects are decisive preconditionsfor their possible development as alternative thin-film solarcells.


PROGRESS

REPORT

www.advmat.de

Figure 7. Band energy diagram as obtained for the CdTe/ZnTe:N interface.Redrawn from [63].

Figure 8. AFM images of the different layers in a CdTe superstrate solarcell. Adapted from [69].

4202

4.2. Control of Nucleation and Growth on Dissimilar

Substrates

In thin-film solar cells, glass, metal or also, most recently,polymer foils are used as substrates. As a consequence, in manycases there is no clear structure relation of the absorber layer tothe many other films needed for completing the solar cell. Theperformance of a thin-film solar cell is thus strongly influenced bylateral inhomogeneities of the deposited layers, whichmay lead tothe formation of recombination pathways along pinholes, grainboundaries, or weak diodes. In addition, the different grainsforming a number of parallel arrays of so-called ‘‘micro solarcells’’ will have different electrical properties.[68] Especially forCdTe solar cells that are prepared in the superstrate configuration,the amorphous-glass substrate or the small-grained TCO layerwill hardly be able to control the texture of the films. In thesecases, nucleation and growth will not be affected by interfacialepitaxy relations but will be more dependent on kinetic growthfactors. The morphologies of the different layers of CdTe solarcells formed in the sequence: glass/TCO/CdS/CdTe and CdCl2activation obtained from atomic force microscopy (AFM)measurements are shown in Figure 8.[69,70]

Two fundamental morphology problems are expected for theformation of solar cells, especially when extremely thin layersclose to the physical limits of about 1–2mmare approached: (i) theinterface between the buffer and the absorber layer is formed withsmall-grained materials emphasizing the contribution of thegrain boundaries (see also [70]; (ii) deep holes between the largergrains of the thicker film may facilitate pinhole formation. Toinvestigate changes in structure and morphology regarding theirdependence on the growth conditions, systematic investigationshave been performed with XRD and scanning electron micro-scopy (SEM).[71–73] XRD experiments indicate there is a strongtendency for the CdTe crystallites to grow with a (111) texture atlower substrate temperatures (see Fig. 9), which is largelyindependent of the substrate used. The strong (111) texture isobserved up to a temperature of�460 8C. For temperatures above520 8C, the preferred orientation changes from (111) to a mixed(220) and (311) distribution of crystal orientations. Interestingly,we also found a drop in the (111) texture at a substratetemperature of �340 8C, which then increases again almost to itsmaximum value. The usually applied growth temperatures for thebest solar cells, at least for CSS deposition, are in thehigh-temperature regime. As is also evident from the SEM


images of the formed films (Fig. 10, side view after cracking of thesample), the different textures also lead to different morphologiesof the films.

CSS growth can thus be divided into three different regimes.In the first regime up to about 320 8C, the grains grow ascolumnar, extended grains in the (111) orientation onto thesubstrate, independent of its crystalline orientation but related insize to the small grains of the TCO/CdS substrate. The formedfilm is very compact, but even cracks are formed in and on thefilm at very-low sample temperatures. In the second growthregime (above 370 8C), the texture is again clearly in the (111)direction, but the grains are considerably larger and have losttheir columnar orientation to the substrate. There is a strongtendency to form pinholes and voids in the growing CdTe films inthis temperature regime. A random grain orientation is observedfor higher deposition temperatures (above 520 8C) that are usuallyused for solar-cell fabrication.[12,74] These CdTe films are formedfrommostly large and compact grains, which, in their lateral size,are comparable to their vertical size. However, due to the differentgrain forms and orientations, there is still a strong tendency toform deep craters and grain boundaries. It is interesting to notethat films formed in the transition regime (340 8C) seem to exposea highly interesting film morphology. The films are very compactwithout pinholes and voids. In addition, the grains seem to berather compact and dense, as is also found for the high-temperature growth regime.

The structural properties and texture as obtained afterpreparation are strongly changed after the CdCl2 activationprocess, as is shown in Figures 9 and 10. The changes in the XRDpatterns obtained after a standard activation process shows thatthe preferential orientation observed after growth is lost.Evidently the activation treatment leads to a recrystallization ofthe deposited film with the added CdCl2 layer acting as a flux, inagreement with other observations. Even when the preferential(111) orientation is lost for all deposition temperatures inagreement with results reported in the literature,[75,76] it is clearthat the texture is not identical for all films, which indicates thatthe number of grains of different orientation depends on theprevious growth history, and is not only dependent on theactivation treatment. This is again evident from the SEM imagesshown in Figure 10 for films grown at different substrate


PROGRESS

REPORT

www.advmat.de

Figure 9. X-ray diffraction patterns (Bragg–Brentano geometry) of as-deposited (left) and activated (right) CdTe thin films. The film thickness is 5–6mm.

Figure 10. SEM cross sections of CdTe films for different substrate temperatures: as deposited(left) and activated (right).

Adv. Mater. 2009, 21, 4196–4206 � 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Wein

temperatures and after the CdCl2 activation.Two growth temperatures seem to produceCdTe films of preferential morphology in closecorrespondence with the SEM picturesobtained before activation. The high-temp-erature regime with substrate temperaturesabove 500 8C, as well as the transition regimeidentified above at �340 8C, seem to producefavourable film textures with compact layersconsisting of large grains of the size of thefilm thickness and with minor concentrationsof voids and craters.

This difference in growth morphology alsoinfluences the electronic properties, as isevident from the performance of solar cellsprepared with the different films. In order torelate the changes in structure and morphol-ogy to changes in the cell performance, solarcells have been produced using the differentfilms with standard conditions, which need tobe a good compromise for the different CdTefilms prepared. For this reason the obtainedenergy-conversion efficiencies are not the bestvalues that can be prepared in our set-up, andare in the range of 10–11% (without using Cuand with a simple Te/Au back contact). Noreasonable conversion efficiencies could beobtained without activation. As expected,maxima in the conversion efficiencies can bereached at elevated temperatures exceeding500 8C. However, rather-similar values areobtained for films grown in the transitionregime of 340 8C. These results suggest thatfilms grown at lower temperatures by CSSmayalso provide good conversion efficiencies if the

heim 4203

PROGRESS

REPORT

www.advmat.de

4204

post-deposition treatment, especially the activation step, is furtheroptimized. On the other hand, films formed at this low substratetemperature seem to provide advantages for the compactness ofthe absorber layer, which would be preferential for thinner solarcells by avoiding pinholes. Successful low-temperature solar-cellpreparation has already been proven by other depositiontechniques, such as magnetron sputtering, PVD and electro-deposition.[32–34] However, the results presented here suggest thatsimilar results are also possible with CSS-based depositiontechniques that have the advantage of their fast deposition rate.

In summary, nucleation and growth of thin-film solar cellsmust be controlled in a better way, as is evidenced from the resultspresented above. The morphology of an optimized absorber layeris sketched schematically in Figure 11, in contrast to non-preferential absorber films as are often obtained in classical CdTegrowth. The aim is to obtain rather compact films, which areevidently favoured at low sample temperatures where recrystalli-zation with the tendency to form 3D grains of similar thicknessand lateral size can be avoided. The high-temperature-depositedgrains tend to form unfavourable recombination pathways alonggrain boundaries or weak diodes, which leads to low shuntresistances and therefore to low conversion efficiencies. However,good electronic properties are only to be expected when growthdefects formed during low-temperature deposition are alsodrastically reduced, which, however, needs sufficiently highannealing temperatures. It may be expected that a combination oflow-temperature-growth with rapid-thermal-annealing steps willallow for bulk-defect recrystallization, but not for completerecrystallization of the grains. The growth is usually performedon a substrate that is not well defined in its structural properties(metal film, glass or TCO layer without preferential structuralorder). As a consequence, there is no tendency to form clearly

Figure 11. Schematic representation of recombination pathways produ-cing low shunt resistances along grain boundaries of three-dimensional,statistically oriented films (left) and the improved morphology of compactCdTe layers expected by improved control of texture and nucleation (right).


definedmicroscopic solar cells with an aligned orientation to eachother and all with similar electronic properties. These micro-scopic solar cells should be formed by aligned single grains withmaximized lateral dimensions and minimized vertical dimen-sions, as given by the absorber lengths. For many compoundsemiconductors this contrasts to the thermodynamically pre-ferred, natural, 3D grain habitus, given by their crystal structures,such as for Zinkblende type materials with face-centered cubicstructures.

5. Summary and Conclusions

Based on the presented results mostly deduced from CdTe solarcells, but also in relation to other thin-film materials, there is astrong need for a more-detailed understanding of process-relatedproperties of thin-film solar cells. Mostly, inhomogeneity effects,which are related to the orientation dependence of growthprocesses starting from badly defined substrates, such as the glasssubstrate, are the dominant contributors to the loss of conversionefficiency. Also, one needs a better understanding of theorientation dependence of the surface and interface propertiesfrom a structural, as well as from an electronic, point of view.Therefore, we feel that despite promising recent results forthin-film solar cells reaching economic competitiveness for CdTeand CIGS, there are still major fundamental and materi-als-science-related questions open for improving these cells inorder to further approach the theoretical limits in efficiency. Inaddition, the lessons learned during optimization of these solarcells and research for improved understanding must betransferred to other materials. CdTe and CIGS solar cells sufferfrom the fact that the available resources of Te and In may not besufficient for a complete and sustainable supply of solarelectricity. Therefore there is an urgent need to investigate anddevelop other interesting materials as absorbers for thin-filmsolar cells. For their development, interface-related propertiesconcerning growth and contact formation must be addressed in aproper way to identify technologically viable solutions. Theelectronic passivation of interface defects must be addressed byappropriate interface engineering strategies. Kinetically con-trolled nucleation and growth measures to deposit compact filmswith a minimum of bulk defects and lateral inhomogeneities areof similar importance. With such measures and using thesuggested device structure shown in Figure 2, one may avoiddoping the absorber materials, which, for many alternativecompound semiconductor materials, also provides unsolvedchallenges.

As the challenges to provide long-lasting and economiccompetitive solutions for solar-electricity generation must be metin a very-short time due to the evident problems of thefossil-carbon-based energy technologies used today, we want tomotivate strongly increased research efforts in advanced conceptsfor thin-film solar cells. As no silver bullet has been identified sofar, many different approachesmust be followed in parallel to findpromising solutions. An extension of the number of promisingabsorber materials is also needed to develop tandem or triple cellsusing thin-film technology. Inorganic thin-film solar cells seem tous to be a very-promising approach, but need more-extendedefforts combining device-related development with fundamentalresearch.


PROGRESS

REPORT

www.advmat.de

Acknowledgements

We would like to thank our coworkers J. Fritsche, D. Kraft, A. Thissen, B.Spath, J. Luschitz, J. Schaffner, B. Siepchen, A. Barati, K. Velappan, G.Haindl, K. Lakus-Wollny, C. Spanheimer, E. Golusda, S. Gottschalk, and J.Brotz who have contributed to the results presented in this paper. Fundingfrom the BMWi and BMU is gratefully acknowledged. The collaborationwith ANTEC and other industrial and scientific institutions is alsoappreciated.

Received: August 22, 2008

Revised: March 3, 2009

Published online: June 24, 2009

[1] A. Goetzberger, V. U. Hoffmann, Photovoltaic Solar Energy Generation,

Springer, Berlin 2005.

[2] European Photovoltaic Technology Platform, http://www.eupvplatfor-

m.org/

[3] B. O’Reagan, M. Gratzel, Nature 1991, 353, 737.

[4] M. Gratzel, J. Photochem. Photobiol. C 2003, 4, 145.

[5] C. J. Brabec, N. S. Sariciftci, J. C. Hummelen, Adv. Funct. Mater. 2001, 11,

15.

[6] M. A. Green, Third Generation Photovoltaics: Advanced Solar Energy Con-

version, Springer Series in Photonics, Vol. 5, Springer, Berlin 2006.

[7] W. Shockley, H. J. Queisser, J. Appl. Phys. 1961, 32, 510.

[8] Handbook of Photovoltaic Science and Engineering, (Eds: A, Luque, S.

Hegedus), John Wiley & Sons, Chichester 2003.

[9] S. Hegedus, Prog. Photovoltaics: Res. Appl. 2006, 14, 393.

[10] M. Powalla, F. Kessler, D. Hariskos, G. Voorwinden, A. N. Tiwari, D.

Bremaud, M. Edoff, S. Schleussner, L. Stolt, B. Dimmler, R. Wachter, R.

Klenk, P. Pistor, D. Abou-Ras, H.-W. Schock, O. Kerrec, P.-P. Grand, D.

Lincot, N. Naghavi, A. Perez-Rodriguez, S. Auvray, presented at 22nd

European Photovoltaic Solar Energy Conference, Milan, Italy September

2007.

[11] D. Bonnet, Thin Solid Films 2000, 361-362, 547.

[12] D. H. Rose, F. S. Hasoon, R. G. Dhere, D. S. Albin, R. M. Ribelin, X. S. Li, Y.

Mahathongdy, T. A. Gessert, P. Sheldon, Prog. Photovoltaics: Res. Appl.

1999, 7, 331.

[13] R. R. King, D. C. Law, K.M. Edmondson, C. M. Fetzer, G. S. Kinsey, H. Yoon,

R. A. Sherif, D. D. Krut, J. H. Ermer, P. Hebert, P. Pien, N. H. Karam,

presented at 22nd European Photovoltaic Solar Energy Conference, Milan,

Italy, September 2007.

[14] B. A. Andersson, Prog. Photovoltaics: Res. Appl. 2000, 8, 61.

[15] Semiconductors Basic Data, 2nd ed, (Ed: O. Madelung), Springer Verlag,

Berlin 1996.

[16] J. Hedstrom, H. Ohlsen, M. Bodegard, A. Kylner, L. Stolt, D. Hariskos, M.

Ruckh, H. W. Schock, presented at 23rd IEEE Photovoltaic Specialists

Conference, Louisville May 1993.

[17] P. Meyers, C. Liu, T. Frey, US Patent 4,710,589 1987.

[18] P. Wurfel, Physics of Solar Cells, Wiley-VCH, Weinheim 2004.

[19] W. Walukiewicz, Physica B 2001, 302–303, 123.

[20] A. Zunger, Appl. Phys. Lett. 2003, 83, 57.

[21] S. M. Sze, Physics of Semiconductor Devices, John Wiley & Sons, New York

1981.

[22] S. B. Zhang, S.-H. Wei, A. Zunger, J. Appl. Phys. 1998, 83, 3192.

[23] Transparent Conductive Zinc Oxide: Basics and Applications in Thin Film Solar

Cells, (Eds: K, Ellmer, A, Klein, B. Rech), Springer-Verlag, Berlin 2008.

[24] W. Jaegermann, A. Klein, J. Fritsche, D. Kraft, B. Spath, Mater. Res. Soc.

Symp. Proc. 2005, 865, F6.1.

[25] M. A. Herman, H. Sitter,Molecular Beam Epitaxy: Fundamentals and Current

Status, Springer Series in Materials Science, Vol. 7, Springer, Berlin 1996.


[26] A. Klein, W. Jaegermann, J. Fritsche, R. Hunger, D. Kraft, F. Sauberlich, T.

Schulmeyer, B. Spath, presented at 31st IEEE Photovoltaic Specialists

Conference, Orlando, Florida, January 2005.

[27] J. Fritsche, D. Kraft, A. Thissen, T. Mayer, A. Klein, W. Jaegermann, Mater.

Res. Soc. Symp. Proc. 2001, 668, H6.6.

[28] F. F. Wang, A. L. Fahrenbruch, R. H. Bube, J. Appl. Phys. 1989, 65, 3552.

[29] D. S. Albin, Y. Yan, M. M. Al-Jassim, Prog. Photovoltaics: Res. Appl. 2002, 10,

309.

[30] J. Fritsche, D. Kraft, A. Thißen, T. Mayer, A. Klein, W. Jaegermann, Thin Solid

Films 2002, 403–404, 252.

[31] N. Romeo, A. Bosio, V. Canevari, A. Podesta, Sol. Energy 2004, 77, 795.

[32] G. Khrypunov, A. Romeo, F. Kurdesau, D. L. Batzner, H. Zogg, A. N. Tiwari,

Sol. Energy Mater. Sol. Cells 2005, 90, 664.

[33] A. Gupta, A. D. Compaan, Appl. Phys. Lett. 2004, 85, 684.

[34] B. E. McCandless, J. R. Sites, in: Handbook of Photovoltaic Science and

Engineering, (Eds: A, Luque, S. Hegedus), John Wiley & Sons, Chichester

2003, 617.

[35] K. D. Dobson, I. Visoly-Fisher, G. Hodes, D. Cahen, Sol. Energy Mater. Sol.

Cells 2000, 62, 295.

[36] D. Grecu, A. D. Compaan, D. Young, U. Jayamaha, D. H. Rose, J. Appl. Phys.

2000, 88, 2490.

[37] D. L. Batzner, A. Romeo, H. Zogg, R. Wendt, A. N. Tiwari, Thin Solid Films

2001, 387, 151.

[38] X. Wu, J. C. Keane, C. DeHart, R. G. Dhere, D. S. Albin, A. Duda, T. A.

Gessert, presented at 17th European Photovoltaic Solar Energy Confer-

ence, Munich, October 2001.

[39] J. Fritsche, A. Thißen, A. Klein, W. Jaegermann, Thin Solid Films 2001, 387,

158.

[40] J. Fritsche, T. Schulmeyer, D. Kraft, A. Thißen, A. Klein, W. Jaegermann,

Appl. Phys. Lett. 2002, 81, 2297.

[41] J. Fritsche, A. Klein, W. Jaegermann, Adv. Eng. Mater. 2005, 7, 914.

[42] B. Siepchen, A. Klein, W. Jaegermann, Phys. Status Solidi (RRL) 2008, 2,

169.

[43] B. E. McCandless, M. G. Engelmann, R. W. Birkmire, J. Appl. Phys. 2001, 89,

988.

[44] D. Schmid, M. Ruckh, F. Grunwald, H. W. Schock, J. Appl. Phys. 1993, 73,

2902.

[45] A. Klein, W. Jaegermann, Appl. Phys. Lett. 1999, 74, 2283.

[46] M. Turcu, O. Pakma, U. Rau, Appl. Phys. Lett. 2002, 80, 2598.

[47] C. Persson, A. Zunger, Phys. Rev. Lett. 2003, 91, 266401.

[48] A. L. Fahrenbruch, R. H. Bube, Fundamentals of Solar Cells: Photovoltaic

Solar Energy Conversion, Academic Press, New York 1983.

[49] J. Meier, S. Dubail, R. Platz, P. Torres, U. Kroll, J. A. A. Selvan, N. P. Vaucher,

C. Hof, D. Fischer, H. Keppner, R. Fluckiger, A. Shah, V. Shklover, K.-D.

Ufert, Sol. Energy Mater. Sol. Cells 1997, 49, 35.

[50] W. Monch, in: Electronic Properties of Semiconductor Interfaces, Springer

Series in Surface Sciences, Springer-Verlag, Heidelberg 2004.

[51] J. Bardeen, Phys. Rev. 1947, 71, 717.

[52] R. H. Bube, Photovoltaic Materials, Series on Properties of Semiconductor

Materials, Vol. 1, Imperial College Press, London 1998.

[53] D. L. Batzner, R. Wendt, A. Romeo, H. Zogg, A. N. Tiwari, Thin Solid Films

2000, 361–362, 463.

[54] L. J. Brillson, S. Chang, J. Shaw, R. E. Viturro, Vacuum 1990, 41, 1016.

[55] I. M. Dharmadasa, Prog. Cryst. Growth Charact. 1998, 36, 249.

[56] A. Klein, F. Sauberlich, B. Spath, T. Schulmeyer, D. Kraft, J. Mater. Sci. 2007,

42, 1890.

[57] S.-H. Wei, S. B. Zhang, Phys. Rev. B 2002, 66, 155211.

[58] B. Spath, K. Lakus-Wollny, J. Fritsche, C. S. Ferekides, A. Klein,

W. Jaegermann, Thin Solid Films 2007, 515, 6172.

[59] H. Wolf, F. Wagner, T. Wichert, Physica B 2003, 340–342, 275.

[60] H. R. Moutinho, R. G. Dhere, C. S. Jiang, T. Gessert, A. Duda, M. Young,

W. K. Metzger, M. M. Al-Jassim, J. Vac. Sci. Technol. B 2007, 25, 361.

[61] K. Kuribayashi, H.Matsumoto, H. Uda, Y. Komatsu, A. Nakano, S. Ikegami,

Jpn. J. Appl. Phys. 1983, 22, 1828.


PROGRESS

REPORT

www.advmat.de

4206

[62] I. Lyubomirsky, M. K. Rabinal, D. Cahen, J. Appl. Phys. 1997, 81, 6684.

[63] B. Spath, J. Fritsche, A. Klein, W. Jaegermann, Appl. Phys. Lett. 2007, 90,

062112.

[64] J. Drayton, A. Gupta, K. Makhratchev, K. J. Price, R. G. Bohn, A. D.

Compaan, Mater. Res. Soc. Symp. Proc. 2001, 668, H5.9.

[65] A. Barati, A. Klein, W. Jaegermann, Thin Solid Films 2009, 517, 2149.

[66] T. Loher, C. Pettenkofer, W. Jaegermann, presented at First World

Conference on Photovoltaic Energy Conversion, Hawaii, December 1994.

[67] M. Roy, S. Damaskinos, J. E. Phillips, presented at 20th IEEE Photovoltaic

Specialists Conf., December 1988.

[68] U. Rau, P. O. Grabitz, J. H. Werner, Appl. Phys. Lett. 2004, 85, 6010.

[69] J. Fritsche, S. Gunst, E. Golusda, M. C. Lejard, A. Thißen, T. Mayer, R.

Wendt, R. Gegenwart, D. Bonnet, A. Klein, W. Jaegermann, Thin Solid Films

2001, 387, 161.


[70] M. A. Cousins, K. Durose, Thin Solid Films 2000, 361–362, 253.

[71] J. Luschitz, B. Siepchen, J. Schaffner, K. Lakus-Wollny, A. Klein,

W. Jaegermann, presented at 33rd IEEE Photovoltaic Specialists Confer-

ence, San Diego, CA, May 2008.

[72] J. Luschitz, B. Siepchen, J. Schaffner, K. Lakus-Wollny, G. Haindl, A. Klein,

W. Jaegermann, Thin Solid Films 2009, 517, 2125.

[73] J. Luschitz, K. Lakus-Wollny, A. Klein, W. Jaegermann, Thin Solid Films 2007,

515, 5814.

[74] D. Bonnet, M. Harr, presented at 2nd World Conference on Photovoltaic

Energy Conversion, Vienna, July 1998.

[75] B. E. McCandless, L. W. Moulton, R. W. Birkmire, Prog. Photovoltaics: Res.

Appl. 1997, 5, 249.

[76] A. Romeo, D. L. Batzner, H. Zogg, A. N. Tiwari, Thin Solid Films 2000,

361–362, 420.


Documents

Interface Engineering of Inorganic Thin-Film Solar Cells ��� Materials-Science Challenges for Advanced Physical Concepts

Interface Engineering of Inorganic Thin-Film Solar Cells �� Materials-Science Challenges for Advanced Physical Concepts